The Characterising Exoplanet Satellite (CHEOPS) is the first mission dedicated to the characterization of exoplanets by means of ultra-high precision photometry on bright stars already known to host transiting planets. It is providing the unique capability of determining accurate radii for a subset of those planets for which the mass has already been estimated from ground-based spectroscopic surveys. CHEOPS can also provide prime targets suited to the spectroscopic characterisation of exoplanetary atmospheres.